Tuesday, June 28, 2011

Is D-Dalus for Real

Via Wolfgang I learned about a purported new type of aircraft supposedly introduced at the Paris air show.  It supposed performance is remarkable:

At the heart of D-DALUS is a revolutionary propulsion system containing a number of patented inventions, including a friction free bearing at the points of high G force, and a system that keeps propulsion in dynamic equilibrium, thereby allowing the guidance system to quickly restore stability in flight.

The propulsion consists of 4 sets of contra-rotating disks, each set driven at the same rpm by a conventional aero-engine. The disks are surrounded by blades whose angle of attack can be altered by off-setting the axis of the rotating disks. As each blade can be given a different angle of attack, the resulting main thrust can be in any required direction in 360° around any axis. This allows the craft to launch vertically, remain in a fixed position in the air, travel in any direction, rotate in any direction, and thrust upwards thereby ‘gluing down’ on landing.

Supposedly it could achieve this ideal:

Ideally we would love an aerial platform that can approach as gently and silently as a hot air balloon, can stay in the air like a humming-bird, can rotate in any direction like a football, can ‘glue down’ on the deck of a ship like a ‘tossed pancake’, can see in all directions like a crystal ball, can fly as fast as a jet, is as invisible as a 155mm shell and can be repaired by a local car mechanic.

I was very puzzled by its principle of operation, but an astute commenter at one site suggested that it was a Voith Schneider propeller, an idea that looks very persuasive to me.  See the animation in the linked Wikipedia article.  (Note that it was an invention of a countryman of Wolfgang).  These have proven very useful for aquatic vehicles.

Whether such a system can work well in air seems less obvious, mainly because of the the 1000 fold smaller mass desnsity of air.

Suppose this vehicle has a mass of 100 kg.  Then it must transfer momentum to ambient air at the rate of d(mv)/dt = mg = 1000 kg m/s^2 in order to produce enough lift to stay aloft.  Air has a density of about 1.3 kg/m^3 (which I round down to 1 kg/m^3 in deference to drag and easy arithmetic) , so if it has a cross section of 5 m^2, and a downward flow stream of air below it of similar dimensions, the downward moving stream of air need a speed of roughly 14 m/s and has a flux of 70 m^3/s.  It seems like a lot of air to collect over a pretty small effective wingspan.